Electroluminescent zinc sulfide of optimum luminosity per gram



July 12, 1966 I. FEUER 3,260,879

ELECTROLUMINESCENT ZINC SULFIDE OF OPTIMUM LUMINOSITY PER GRAM Filed May 26, 1965 FIG. I.

FIG. 2.

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ATTORNEYS.

United States Patent 3,260,879 ELECTROLUMINESCENT ZINC SULFIDE OF OPTIMUM LUMINOSITY PER GRAM Irving Feuer, Elmhurst, N.Y., assignor to Canrad Preclsion Industries, Inc., New York, N.Y., a corporation of New York Filed May 26, 1965, Ser. No. 461,600 4 Claims. (Cl. 313-108) This application is a continuation-in-part of my copending application Serial No. 191,920, filed May 2, 1962 and now abandoned.

This invention relates to electroluminescent phosphors as may be used in electroluminescent phosphor lamps.

Electroluminescent phosphor lamps are made up of a condenser in which one of the electrodes is transparent and in which there is interposed between the electrodes a dielectric having electroluminescent phosphor dispersed therein. Upon application of an alternating voltage to the condenser electrodes, the phosphor lurninesces, and the light is visible through the transparent electrode. Such lamps are sold in commerce, but as yet have not come into widespread use. More general use of the lamps awaits improvement in various respects and especially improvement which will provide increased luminosity with respect to applied voltage, and improvement in respect to-the life and appearance of the lamps, in the sense that breakdown of the dielectric layer and discolorations which commonly appear at the transparent electrode are eliminated or reduced. Principal objects of the invention are to provide improvements in voltage versus luminosity, and in regard to breakdown and discoloration.

As to voltage versus luminosity, it is an object of the invention to provide for increased luminosity per unit weight of phosphor, whereby for a given impressed voltage, increased luminosity can be obtained, or, alternatively, a reduced voltage can be used to obtain a given luminosity per unit weight of phosphor. Otherwise stated, luminosity per gram of phosphor per volt of applied volttage is to be increased, and, for-a given luminosity per gram, there is a reduced voltage drop per particle.

Another object of the invention is to reduce the threshold voltage of electroluminescent lamps per unit weight of phosphor. By threshold voltage, is meant the lowest voltage at which the lamp becomes noticeably luminescent. Threshold voltage is reduced by improving voltage versus luminosity per gram, and thus the invention is directed to improvement in both these respects.

With respect to breakdown and discoloration, breakdown of the layer interposed between the electrodes occurs and portions of the interposed layer become inoperative to provide luminescence. This breakdown is believed to be a consequence of direct path or substantially direct path through phosphor particles from one electrode to the other. Arcing may then occur with the result that the phosphor does not operate in the intended manner and a portion of the dielectric layer becomes inoperative to provide luminescence. The breakdown likely results from insufiicient isolation of the phosphor particles from one another by the dielectric. The invention provides for dispersion of the phosphor in the dielectric in a manner such that breakdown does not occur, or the occurrence of breakdown is substantially less frequent.

With respect to discolorations, commonly discolorations appear over small but visible areas of the transparent electrode. The discolorations are unsightly and reduce the luminescence of the lamps. It is believed that the discolorations are the result of decomposition reactions occurring in the layer interposed between the electrodes and adjacent the transparent electrode. It seems likely that decomposition of the dielectric is the 3,260,879 Patented July 12, 1966 cause, and that this occurs upon arcing in the condenser, adjacent the transparent electrode.

The'invention provides for a structure of the layer in- I terposed between the electrodes such that discolorations do not occur, or such that the occurrence of discolorations is substantially less frequent.

The manner in which the foregoing objectives and still other objectives are obtained, will be apparent from the following description in reference to the accompanying drawings, of which:

FIG. 1 is a view of an electroluminescent lamp; and

FIG. 2 is a graph of luminosity per gram versus particle size, the particle size being expressed as grinding time.

The improvement according to the invention whereby there is obtained reduction in breakdown in the dielec tric layer and reduction in discoloration of the transparent electrode, is realized by using phosphor particles having substantially all of their diameters below 2 microns wherein at least about of the phosphor particles have diameters of below about 0.8 micron and at least about of the phosphors have diameters of above about 0.05 micron with the proviso that at least 5% of the phosphor particles have diameters of greater than 0.8 micron and less than 2 microns. While the reason for reduction in breakdown and discoloration is not surely known, it is likely that these improvements result from better dispersion of the phosphor in the dielectric, in the respect that the extent to which the particles are covered by the dielectric and eifectively isolated from each other is greatly increased. As to breakdown, it is likely that because of the smaller particles size, the occurrence of a direct or substantially direct path through phosphor particles from one electrode to the other, is substantially reduced or eliminated, so that breakdown would be reduced; and that because of better dispersion and coating of the particles, contact between the particles and each of the electrodes is reduced or eliminated so that a direct path through phosphor between the electrodes is prevented. As to discoloration, it is likely that reduction in discoloration is occasioned by reason of better covering of the particles preventing contact between the phosphor particles and the transparent electrode.

With respect to improvement in voltage versus luminescence per gram, here again the improvement is realized by utilizing phosphors having the aforementioned particular particle size distribution. To facilitate understanding of this aspect of the invention, consideration of FIG. 2 is desirable. As stated above, FIG. 2 of the drawing is a plot of luminescence per gram versus particle size, the particle size being measured and expressed quantitatively as the time during which the phosphor is subjected to size reduction or grinding. Curve 1 and curve 2 are for different phosphors. The starting material for these curves was derived by the commonly practiced prior art procedure of firing a suitably constituted mixture of powders, and then breaking up the glomerates. The starting material of curve 1 had phosphors of which at least 99% of the diameters of the particles were greater than 1 micron; the starting material of curve 2 had phosphors of which at least 50% of the diameters of the particles were greater than 1 micron. As such, these two phosphors are of particle size as have been heretofore commonly used in the art. The treatment to which the starting phosphors were subjected involved ball milling and is described in detail in the examples set forth below. Luminosity per gram is in relative units and in the case of each of the curves, the luminosity per gram of the starting material is taken as 1.

As seen from FIG. 1, as grinding of the particles proceeds there is an increase in luminosity of the phosphor. However, after the grinding has proceeded for a while, there is a decrease in the luminosity of the phosphor until the luminoisty decreases to a value below the value it had before grinding. This increase in luminosity, it has been found, occurs by means of the fact that the particle sizes or diameter of the phosphor particles are in a range which produces maximum lminescence. This range occurs when substantially all of the particles have diameters of less than 2 microns and greater than about 0.05 micron and at least about 80% of the phosphors have particle sizes or diameters below about 0.8 micron with the proviso that at least of the phosphors have particle sizes between 0.8 micron and 2 microns. As shown in curve 1 of FIG. 2, the luminosity is decreased by grinding to the original luminosity at grinding time of 29 hours. By analyzing samples of the phosphors at this grinding time, it was found that the diameters of the phosphor particles had been reduced to a value wherein less than about 5% of the particles had diameters of greater than about 0.8 micron. By also analyzing a sample of the phosphor particles of curve 2 of FIG. 1 after 38 hours of grinding (where the luminosity is equal to 1) was found that at that grinding time, the particle diameters had just been reduced so that less than 5% of the particles had diameters of between about 0.8 micron and about 2.0 microns. Furthermore, in order to produce the improved results of this invention, it is necessary that at least 95% of the particles have diameters of above about 0.05 micron. This is true, since, very little decrease in luminosity or other properties occurs when less than about 5% of the particles has minute or tiny particle diameters or sizes since this small number has little effect on the properties of the rest of the phosphor particles. However, once a greater portion of the phosphor particles have diameters of below 0.05 micron, their properties will be adversely affected. This is evidence of the fact that after the luminosity in curves 1 and 2 reaches 1, continued grinding will cause the luminosity to be reduced further.

From the foregoing, it will be seen that the use of a particle size according to the invention provides increased luminosity per gram.

Further, the threshold voltage is reduced in accordance with improvement in luminosity, and hence the considerations applying to the improvement in luminosity per gram apply also to the reduction in threshold voltage. I FIG. 1 of the accompanying drawing depicts an electroluminescent condenser. The condenser includes a base electrode 3, a transparent electrode 4, and, interposed between these electrodes, a layer which is composed of a dielectric having dispersed therein, preferably substantially uniformly, phosphor particles of particle size according to the invention.

Commonly, the layer 5 will be in contact with each of the electrodes, though the electrodes may have a film of material interposed between the dielectric layer and the electrode, the film being for the purpose of improving the electrode operation in various effects, for example further improving the electrode with respect to the occurrence of discoloration of the transparent electrode.

Electrically connected to the condenser shown in the drawing, is an alternating current source 6, and upon application of an alternating voltage, luminescence occurs and light is visible through the transparent electrode.

Providing the phosphor of particle size according to the invention can be accomplished by known means for size reduction, e.g. by grinding, or can be by the growing of crystals, as can be effected by procedures involving precipitation of the phosphor.

The phosphors with which the invention is concerned are all those materials which, when utilized as described above for phosphor with reference to FIG. 1, luminesces. The material can be phosphor of the zinc sulfide type, particularly those containing a small amount of copper as impurity.

Similarly, the dielectric can be any dielectric suitable for use in electroluminescent cells. For example, the dielectric can be castor oil, dibutyl phthalate, Cyanocell brand cyanoethylated cellulose which may be mixed with a plasticizer and/ or other dielectric as is known in the art, epoxy dielectric, dimethyl phthalate, or vinyl dielectric.

Particle size is expressed herein as milled particle size, by which is meant that there can be a particle size distribution corresponding to that obtained when particle size is effected by milling or grinding. As has been indicated hereinbefore, it is not necessary to the invention that the size reduction be effected by milling or grinding. Particle sizes reported herein were measured with a Coulter Counter in a 1% sodium chloride solution, and thus the values are for mean particle diameter statistically determined.

The invention is further described in the following examples which are for the curves shown in FIG. 2, curve 1 being for Example 1, and curve 2 for Example 2.

EXAMPLE I The starting material for this example is zinc sulfide type phosphor available commercially under the trademark Diwit 202, a product of Diehl, Germany. This phosphor is 66% ZnS, Cu activated, 33.6% Mns and is an orange electroluminescent phosphor.

The milled particle size of the starting material for the procedure of the invention is as follows:

Percent Greater than 1 micron 99 15 microns 24 5-20 microns 75 Fifty (50) grams of this material is ball milled in a mill 60 mm. inside diameter, and operated at 278 rpm.

The lining of the drum and the balls is Pyrex brand Milling was continued for 36 hours and samples of the material were periodically withdrawn and examined for electroluminescence. Curve 1 of FIG. 2 is a plot of luminescence versus grinding time for this example.

The phosphor samples were used to make cells using Cyanocell as the dielectric in the proportioin of 1 part Cyanocell to 10 parts ZnS and a dielectric film or layer 0.2 mil in thickness was formed between a conventional transparent glass electrode and a conventional metalized electrode. The film was interposed between the electrode in a conventional manner by film knife technique (Gardner). A voltage of 110 volts at 60 cycles was impressed and luminosity readings were made with an Aminco photometer. The luminosity per gram value for the nnmilled (before grinding according to the invention) phosphor was arbitrarily taken as 1. The luminosity for the unmilled material was about 0.3 foot lamberts.

The luminosity after twelve hours was about 1 foot lambert.

The particle size after twelve hours was 97% less than 1 micron and the distribution was as follows:

Percent Less than 2 microns 100 Less than 1 micron 97 Less than 0.8 micron 92.5 Less than 0.4 micron 80.0 Greater than 0.08 micron 99.0

Particle size was determined with a Coulter Counter in a 1% NaCl solution.

Other film or dielectric layer thicknesses ranged from 0.1-3 mils, and voltages over the range 6-150 volts, at 60-1000 cycles, were used. The improvement according to the invention is realized for these various conditions, and is not dependent on frequency.

EXAMPLE II The starting material for this example is a blue zinc sulfide type phosphor made as follows:

Pure ZnS is prepared by precipitating ZnS from an ammoniacal zinc salt solution. The purified ZnS in amount of 5000 grams is formed into an aqueous slurry containing 0.4-0.6 gram of cupric chloride and grams of magnesium chloride. After thorough stirring, the slurry is evaporated to dryness. The dry residue is fired between 10001300 C. for 1-3 hours, cooled, and ground to 200 nylon mesh. The ground material is mixed with cupric chloride aqueous solution, evaporated to dryness, fired at approximately 1100 C. for %1 to 2 hours, washed with cyanide solution, sieved, and tested for luminosity per gram. The resulting material is the starting material for the invention. It is milled in accordance with the invention, and as in Example I, to a submicron particle size.

The particle size distribution of the said starting material Was as follows:

Percent Greater than 0.5 micron 99 Greater than 1 micron 58 From 1-5 microns 32 From 5-20 microns 7.5

This material was ball milled as described in Example I, but for 48 hours. Samples were taken and tested for luminosity per gram as. described in Example I, and curve 2, FIG. 2 sets forth the results of such testing.

A sample taken after 20 hours grinding had about maximum luminosity. This material had a particle size distribution as follows:

Percent Less than 2 microns 100 Less than 1 micron 83 less than 0.8 micron 79 Less than 0.4 micron 67 Greater than 0.8 micron 99 From the foregoing, it will be seen that the invention provides a material for electroluminescent lamps which permits the obtaining of greater luminescence per gram for a given voltage, or for the use of a reduced voltage for a given luminescence, and that these advantages are realized by utilizing phosphor of particle size according to the invention. 7

According to the present invention, luminosity per gram of phosphor increases as particle size decreases.

It is to be noted that the benefits in terms of reduction in occurrence of breakdown and in reduced discoloration are improvements applicable for phosphors generally. These last-mentioned benefits mean longer lamp life and are obtained by using material having the claimed size range. As indicated in FIG. 2, the particle size should not be so small that luminosity per gram is lowered to any objectionable extent. Thus, the luminosity per gram should not be substantially less than the luminosity per gram of the 95% less than 0.8 micron.

A further advantage which can be realized by utilizing small particle size phosphor according to the invention, is that the packing fraction utilized in the condensers can be increased. That is, the Weight of phosphor dispersed in a given amount of dielectric can be increased. On a weight basis, in the prior art, the ratio of phosphor to dielectric is about 1 to 1, Whereas according to the invention the ratio is preferably about 3-1 or higher, e.g. 20-1. According to the invention, there are more particles per unit volume. Still another advantage is that by utilizing a particle size according to the invention, thinner dielectric layers can be used.

The invention is particularly useful when utilized in connection with low voltage electroluminescent lamps, for example 60-100 cycle, 6-30 volt lamps.

Unless otherwise indicated, particle sizes given herein are in terms of number of particles. Thus, 80% less than 0.8 micron means that of the particles are less than 0.8 micron. The data herein on particle size is for milled or ground material, and accordingly, for this data, the distribution of particles in respect to size is as is obtained by milling or grinding.

While the invention has been described with reference to particular embodiments thereof, it is intended to secure by these Letters Patent all such variations, modifications and alterations as will be apparent to those skilled in the art from the appended claims.

What is claimed is:

1. A composition of matter consisting essentially of an electroluminescent zinc sulphide phosphor particles substantially uniformly dispersed in dielectric, substantially all of said particles having diame-ters below 2 microns which at least about 80% of the phosphor particles having diameters below about 0.8 micron and at least of the phosphor particles having diameters of above about 0.05 micron with the proviso that at least 5% of the phosphor particles have diameters of greater than about 0.8 micron and less than about 2 microns, said phosphor having increased luminosity per gram as compared with coarser phosphor particles.

2. An electroluminescent condenser comprising an electrode, a layer of electroluminescent zinc sulphide phosphor particles and dielectric material over said electrode, said zinc sulphide phosphor particles being dispersed in said dielectric material and a transparent electrode over the layer of phosphor and dielectric material, substantially all of said particles having diameters below 2 microns with at least about 80% of the phosphor particles having diameters below about 0.8 micron and at least 95 of the phosphor particles having diameters of above about 0.05 micron with the proviso that at least 5% of the phosphor particles have diameters of greater than about 0.8 micron and less than about 2 microns, said phosphor having increased luminosity per gram as compared with coarser phosphor particles.

3. In the method of making an electroluminescent condenser, the improvement consisting of interposing a layer of dielectric having zinc sulphide electroluminescent particles dispersed therein between the electrodes, substantially all of said particles having diameters below 2 microns with at least about 80% of the phosphor particles having diameters below about 0.8 micron and at least 95 of the phosphor particles having diameters of above about 0.05 micron with the proviso that at least 5% of the phosphor pa-rticles have diameters of greater than about 0.8 micron and less than about 2 microns, said phosphor having increased luminosity per gram as compared with coarser phosphor particles.

4. The method of increasing the luminosity per gram of electroluminescent zinc sulphide phosphor particles which comprises reducing the particle size of said zinc sulphide phosphor particles for a sufiicient time so as to recover a zinc sulphide phosphor particle fraction wherein substantially all of said particles having diameters below 2 microns with at least about 80% of the zinc sulphide phosphor particles having diameters below about 0.8 micron and at least 95 of the phosphor particles having diameters of about 0.05 micron with the proviso that at least 5% of the zinc sulphide phosphor particles have diameters of greater than about 0.8 micron and less than about 2 microns, said phosphor having increased luminosity per gram as compared with coarser zinc sulphide phosphor particles.

References Cited by the Examiner UNITED STATES PATENTS 2,957,830 10/1960 Goldberg et al 252-3015 3,040,201 6/1962 Lehmann 313-108 3,040,202 6/1962 Lehmann 252-301.6 3,048,731 8/1962 Lehmann 313-108 TOBIAS E. LEVOW, Primary Examiner. R. D. EDMONDS, Assistant Examiner. 

1. A COMPOSITION OF MALTTER CONSISTING ESSENTIALLY OF AN ELECTROLUMINESCENT ZINC SULPHIDE PHOSPHOR PARTICLES SUBSTANTIALLY UNIFORMLY DISPERSED IN DIELECT, SUBSTANTIALLY ALL OF WHICH PARTICLES HAVING DIAMETERS BELOW 2 MICRONS WHICH AT LEAST ABOUT 80% OF THE PHOSPHOR PARTICLES HAVING DIAMETERS BELOW ABOUT 0.8 MICRON AND AT LEAST 95% OF THE PHOSPHOR PARTICLES HAVING DIAMETERS OF ABOVE ABOUT 0.05 MICRON WITH THE PROVISO THAT AT LEAST 5% OF THE PHOSPHOR PARTICLES HAVE DIAMETERS OF GREATER THAN ABOUT 0.8 MICRON AND LESS THAN ABOUT 2 MICRONS, SAID PHOSPHOR HAVING INCREASED LUMINOSITY PER GRAM AS COMPARED WITH COARSER PHOSPHOR PARTICLES. 